Fix oilpan build breakage from r176803.

culprit CL: http://crrev.com/338833007

The oilpan return types were not consistent.

BUG=378670
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/352553002

git-svn-id: svn://svn.chromium.org/blink/trunk@176808 bbb929c8-8fbe-4397-9dbb-9b2b20218538